Persons unaware of their HIV infection contribute disproportionately to ongoing HIV transmission. Treatment as prevention (TasP) depends on reaching HIV-infected persons for HIV testing and diagnosis, linking them to care, and initiating antiretroviral therapy. TasP effectiveness may be reduced if a significant proportion of ongoing transmission is related to acute or recent HIV infection, but acute HIV infection (AHI) presents diagnostic challenges. Identifying persons with AHI and recent infection may be critical for the realization of the full potential of TasP. Partner notification is an underused strategy in sub-Saharan Africa to identify persons with undiagnosed HIV infection. Contract partner notification is a cost-efficient form of partner notification, in which newly diagnosed HIV-infected persons are given a limited time to notify their sexual partners, followed by active tracing by health personnel. Social contact referral involves referral of peers for HIV testing. The rationale for ths approach is that social contacts of HIV-infected persons are likely to share similar high-risk behaviors, thereby increasing their likelihood of HIV infection. Our central hypotheses are: 1) Partner notification and social contact referral of newly identified HIV- infected STI clinic patros will effectively identify HIV-infected-but-unaware persons at high risk for ongoing transmission, and 2) HIV-infected unaware sexual partners and social contacts of persons with acute and recent HIV infection will have greater potential for HIV transmission, based on sexual behavior, viral load, and cluster membership, than unaware sexual partners and social contacts of chronically HIV-infected persons. We will address these hypotheses in Lilongwe, Malawi with three specific aims: Aim 1: To evaluate an unaware intervention package, including detection of AHI, contract partner notification, and social contact referral, to find persons unaware of thei HIV infection; Aim 2: To characterize the potential for ongoing HIV transmission through evaluation of sexual behaviors, awareness of HIV status, viral load, and phylogenetic clustering among acute, recent, and chronically HIV-infected STI clinic patrons and their sexual partners and social contacts; Aim 3: To assess the potential for AHI detection, contract partner notification, and social contact referral to reduce HIV incidence using mathematical modeling. To address these aims, we will recruit HIV-infected persons in two sexually transmitted infection clinics in Malawi. We will assess an unaware intervention package combining AHI detection, contract partner notification, and social contact referral. We will stage the HIV infection of inde participants, sexual partners and social contacts using HIV serology, HIV RNA PCR, a multi-assay algorithm, and an exploratory deep sequencing approach. Phylogenetic linkage and cluster analyses will be performed for all identified infections. Mathematical modeling will be used to assess the potential population-level impact of these interventions.